1. Field of the Invention
The present invention relates to gas turbines and more particularly to steam-augmented gas turbines.
2. Description of the Related Art
Many heat-recovery schemes have been proposed in prior art. For example, the so-called "combined cycle", in widespread use today, is an arrangement that improves efficiency with the use of a steam cycle in series with a combustion-turbine cycle. Heat contained in a hot gas leaving an expander is recovered to generate high-pressure steam, which in turn drives a steam turbine. A high efficiency is attained in the combined cycle when the hot gas entering the expander is around 1260.degree. C. (2300.degree. F.).
Another scheme in limited use today is the so-called steam-injected cycle in which high-pressure steam is injected into a combustor. U.S. Pat. No. 5,329,758 to Urbach et al. discloses a steam-augmented gas turbine (SAGT) engine employing a heat recovery steam generator (HRSG). In order to obtain maximum efficiency of the steam turbine, it is desirable to generate steam at a high temperature and pressure. However, the HRSG limits the quantity of the steam generated because of a heat recovery limitation. More specifically, the HRSG operates at greatly reduced efficiency, since the recovery liquid approaches and exceeds it's saturation temperature and pressure causing the temperature difference between the exhaust gases and the recovery liquid to be minimized. This phenomenon limits the maximum achievable boiler pressure and the minimum achievable exhaust gas temperature putting a limit to the possible improvement in efficiency.
U.S. Pat. No. 5,564,269 to Briesch proposes to overcome the limitation in thermal efficiency of the SAGT engine by utilizing an improved HRSG that generates steam at multiple pressure levels by employing a separate evaporator at each pressure level. Although this system increases the recovery of heat from the exhaust gas, the HRSG is complex, involving large sized steam drums, a large duct, etc., and require a considerable capital investment.
Despite the potential advantages of the Urbach et al. '758 and Briesch '269 systems none of these systems are cost effective, compact in structure and high in thermal efficiency. Firstly, the turbine input temperatures are maintained at high values in the range of 1204.degree. to 1370.degree. C. (2200.degree.-2500.degree. F.) in order to obtain the maximum thermal efficiency and such higher input temperatures would require much more expensive material for the construction of the turbine blades and the heat recovery boiler. At high turbine input temperatures, secondly, blade cooling is necessary and the turbine structure becomes complex. In spite of the high input temperatures, further, the thermal efficiency of such engines does not increase because the air compressor should compress more air than is needed for combustion to provide excess air to be used for cooling the blades. The energy used to compress the excess air is a parasitic load on the turbine and this load has a major impact on the thermal efficiency. For example, one-half to two-thirds of the power produced by the turbine is used to drive the air compressor, thus leaving only about one-third to one-half of the power available for useful work. Thirdly, the SAGT systems discussed above consume large amount of water since the exhaust gases containing a large amount of steam are rejected from the HRSG without recycling. Thus, in the SAGT system employed in the ship disclosed in Urbach et al. '758, a large scale water purification system composed of an expensive reverse osmosis desalinator is required and the water purification system occupies a large space in a limited space of the ship. Further, since the output shaft of the turbine rotates at an extremely high speed, the SAGT systems are equipped with a large and heavy reduction gear unit which shares a relatively large space in a limited area of the ship.